Fender Showman '68

vic

Got this Showman yesterday. It's an early '68 with the AB763 (blackface) circuit. It's rated at 8 ohm and 85 watt according to literature (thought it would be 4 ohm like the BM or Bandmaster).

Problem is that I have a '62/63 vintage 4 ohm Fender 2*12 cab (Oxford speakers) and I guess it won't be a good idea to pair these two....? Any advice ? I noticed here http://ampwares.com/amplifiers/fender-silverface-showman/ that Showmans (8ohm) had 4 ohm cabs ???

JeremyE

Hey vic

Looks like the head is in very nice condition! The showman had 2 different cabs, the first (Showman 15) containing a 15 inch 8ohm speaker and the second cab (Dual Showman) containing 2 of these 15inch 8ohm speakers wired in parallel (so it becomes 4ohms). My guess (as I haven't researched this due to being at work) is that the speakers could handle the power output of the amp despite the impedance miss-match.
Basically what happens is that when the 8ohm head see's the 4ohm cab the power into the cab doubles so the speakers would need to be able to handle that.

If your cab is designed for a bassman or bandmaster is it most likely designed to handle 50watts at 4ohms so connecting the head and pushing the volume may very well cause some speaker damage. Hope this helps...

vic

JeremyE wrote: Hey vic

Looks like the head is in very nice condition! The showman had 2 different cabs, the first (Showman 15) containing a 15 inch 8ohm speaker and the second cab (Dual Showman) containing 2 of these 15inch 8ohm speakers wired in parallel (so it becomes 4ohms). My guess (as I haven't researched this due to being at work) is that the speakers could handle the power output of the amp despite the impedance miss-match.
Basically what happens is that when the 8ohm head see's the 4ohm cab the power into the cab doubles so the speakers would need to be able to handle that.

If your cab is designed for a bassman or bandmaster is it most likely designed to handle 50watts at 4ohms so connecting the head and pushing the volume may very well cause some speaker damage. Hope this helps...

Thanks Jeremy, much appreciated. However I am more concerned about damaging the amp (8 ohm amp into 4 ohm speakers).

JeremyE

Small update... The Dual showman cab was designed for a Dual Showman head having 2 4ohm outputs (to drive 2 cabs). Hence the 4ohm Dual showman cab.

Basically running the 8ohm head into the 4ohm cab you'll get greater power output, but risk burning out the output transformer and/or other components...

vic

JeremyE wrote: Small update... The Dual showman cab was designed for a Dual Showman head having 2 4ohm outputs (to drive 2 cabs). Hence the 4ohm Dual showman cab.

Basically running the 8ohm head into the 4ohm cab you'll get greater power output, but risk burning out the output transformer and/or other components...

Exactly...that's my fear. Unless the Showman OT can handle a 4ohm load ? Yes, the dual Showman was a 4 ohm head.

bottledtone

The output transformer is just a load matching device! It matches the impedance of the speakers with the load the output valves see. So for example, the 2 X 6L6's want to see a 4K load to run efficiently, and when matched with a incorrect speaker load they will a different load and run inefficiently (4ohm speaker into 8ohm tap = 2K load on valves/ 16 into 8ohm tap = 8K load on valves. So less power!! slighty different tone.
No big problem with this type of mismatch. Just don't go extreme.

AlanRatcliffe

We've had this kind of question before and the consensus from the knowledgeable amp builder types was that lower speaker load is OK to half the rated impedance. Unlike solidstate amps, it doesn't increase the amp output and drive the amp harder because the transformer coupling keeps the output constant.

Higher speaker loads are more problematic because the speaker does not use all the available power, and the unused power builds up in the transformer.

Probably a simplistic explanation, but good enough for me... ?

[EDIT: Bottledtone beat me to it...]

vic

Alan Ratcliffe wrote: We've had this kind of question before and the consensus from the knowledgeable amp builder types was that lower speaker load is OK to half the rated impedance. Unlike solidstate amps, it doesn't increase the amp output and drive the amp harder because the transformer coupling keeps the output constant.

Higher speaker loads are more problematic because the speaker does not use all the available power, and the unused power builds up in the transformer.

Probably a simplistic explanation, but good enough for me... ?

[EDIT: Bottledtone beat me to it...]

Many thanks guys....I've done quite a bit of reading today on various forums and could not find any definitive answer. Some say it's OK to go up... say 8ohm amp into 16ohm speaker cab... and not OK to go down....., others say it's OK to go down....say 8ohm amp into 4 ohm speaker cab, but not up ....grrrr :-\ :-\

bottledtone

vic wrote:
Alan Ratcliffe wrote: We've had this kind of question before and the consensus from the knowledgeable amp builder types was that lower speaker load is OK to half the rated impedance. Unlike solidstate amps, it doesn't increase the amp output and drive the amp harder because the transformer coupling keeps the output constant.

Higher speaker loads are more problematic because the speaker does not use all the available power, and the unused power builds up in the transformer.

Probably a simplistic explanation, but good enough for me... ?

[EDIT: Bottledtone beat me to it...]
Many thanks guys....I've done quite a bit of reading today on various forums and could not find any definitive answer. Some say it's OK to go up... say 8ohm amp into 16ohm speaker cab... and not OK to go down....., others say it's OK to go down....say 8ohm amp into 4 ohm speaker cab, but not up ....grrrr :-\ :-\

As long as you don't go far from the desired ohmage! It is safer to put a 4ohm into an 8ohm output because the load on the valves will be less. It is very dangerous to have no speaker connected which would be a huge open load on the output valves 100000000000kohm eg, and having a 16ohm+ speaker in say a 4ohm output on an amp is closer to having an open load on the valves. Not even near open but would be closer

vic

bottledtone wrote:
vic wrote:
Alan Ratcliffe wrote: We've had this kind of question before and the consensus from the knowledgeable amp builder types was that lower speaker load is OK to half the rated impedance. Unlike solidstate amps, it doesn't increase the amp output and drive the amp harder because the transformer coupling keeps the output constant.

Higher speaker loads are more problematic because the speaker does not use all the available power, and the unused power builds up in the transformer.

Probably a simplistic explanation, but good enough for me... ?

[EDIT: Bottledtone beat me to it...]
Many thanks guys....I've done quite a bit of reading today on various forums and could not find any definitive answer. Some say it's OK to go up... say 8ohm amp into 16ohm speaker cab... and not OK to go down....., others say it's OK to go down....say 8ohm amp into 4 ohm speaker cab, but not up ....grrrr :-\ :-\

As long as you don't go far from the desired ohmage! It is safer to put a 4ohm into an 8ohm output because the load on the valves will be less. It is very dangerous to have no speaker connected which would be a huge open load on the output valves 100000000000kohm eg, and having a 16ohm+ speaker in say a 4ohm output on an amp is closer to having an open load on the valves. Not even near open but would be closer

Are you saying it's OK to hook up a 4ohm cab to the 8ohm Showman head ?

Gearhead

I think Bottledtone is saying yes, but I am sure I would say so. No problem in halving the impedance, if insecure then feel the tranny temperature.

vic

Thanks guys.

bottledtone

Gearhead wrote: I think Bottledtone is saying yes, but I am sure I would say so. No problem in halving the impedance, if insecure then feel the tranny temperature.

haha, well let me put it into other words then. There are many amps out there that will have wattage options. eg. #1: A matchless dc30 which you can switch to 15watts by removing two valves. #2: A mesa mark1 and up, which takes two valves out the circuit, #3: A dr z Remedy, Jaz20/40 etc that removes two valves from the circuit on on on and on........
When these valves are out the circuit, the orginal impedance remains the same, which means it is now mismatched to the speakers....... no kaboom ?
Even different manufacturers use different loads on the output valves, and even within the models of one manufacturer eg. deluxe reverb 6k load vs princeton reverb 8k load.
A small mismatch is no problem..... valves are soooo forgiving. (just for interest, EL34's are normally run at 450v with a load of 3k4, but can be run at 800v with a load of 11k for an output of 100watts!!)

vic

bottledtone wrote:
Gearhead wrote: I think Bottledtone is saying yes, but I am sure I would say so. No problem in halving the impedance, if insecure then feel the tranny temperature.
haha, well let me put it into other words then. There are many amps out there that will have wattage options. eg. #1: A matchless dc30 which you can switch to 15watts by removing two valves. #2: A mesa mark1 and up, which takes two valves out the circuit, #3: A dr z Remedy, Jaz20/40 etc that removes two valves from the circuit on on on and on........
When these valves are out the circuit, the orginal impedance remains the same, which means it is now mismatched to the speakers....... no kaboom ?
Even different manufacturers use different loads on the output valves, and even within the models of one manufacturer eg. deluxe reverb 6k load vs princeton reverb 8k load.
A small mismatch is no problem..... valves are soooo forgiving. (just for interest, EL34's are normally run at 450v with a load of 3k4, but can be run at 800v with a load of 11k for an output of 100watts!!)

I understand that...no problem. BUT the output tranny in the examples you mention is still wound to those specs that will take care of the impedance of the speaker/s. IOW, I'm concerned about damage to the OT (and not really to the valves) when a mismatched speaker cab (in my case a lower impedance) is used ? Anyway, thanks a lot Bottledtone ?